The Blue Moon Explained

A full Moon
The Moon isn't blue, but the phenomenon of a "Blue Moon" (meaning an extra full Moon in a given period), is very real. NASA

"Once in a blue moon."

You've probably heard this expression before, but might not know what it means. It's a pretty common saying. Most people know that it really doesn't mean that the Moon (our closest neighbor in space) actually turns a bluish color. You can see just by looking that the Moon's surface is actually a dull grey. In the sunlight, it appears a bright yellow-white color, but it never turns blue.

 So, what's the big deal with "blue moon"?

It's a Figure of Speech

The term really is a kind of "code meaning "not very often" or "something very rare". It may have begun with a little-known poem written in 1528, Read me and be not wrothe, For I say no things but truth:

"If they say the moon is blue,
"We must believe that it is true."

Calling the Moon blue was an obvious absurdity, like saying it was made of green cheese or that it has little green men living on its surface.  The phrase, “until a blue moon” developed in the 19th century, meaning "never", or at least "extremely unlikely." 

Another Way to Look at the Idea of Blue Moon

A blue Moon is more familiar as a slang term for an actual astronomical phenomenon. That usage first started in 1932 with the Maine Farmer’s Almanac. Its definition included a season with four full Moons rather than the usual three, where the third of four full Moons would be called a "blue Moon." Since seasons are established by the equinoxes and solstices and not calendar months, it is possible for a year to have twelve full Moons, one each month, yet have one season with four.


That definition mutated into the one most quoted today when in 1946, an astronomy article by amateur astronomer James Hugh Pruett misinterpreted the Maine rule to mean two full Moons in one month. This definition seems to have stuck, despite its error, possibly thanks to being picked up by the Trivial Pursuit game.

Whether you use the newer definition or the one from the Maine Farmer’s Almanac, a blue Moon, while not common, does happen pretty regularly. You can expect to see one about seven times in a 19-year period.

Much less common is a double blue Moon (two in one year). That only happens once in the same 19-year period.The last set of double blue Moons happened in 1999. The next ones will happen in 2018.

Can the Moon Appear to Turn Blue?

Normally in the course of a month, the Moon doesn't turn blue itself. But, it can look blue from our vantage point on Earth due to atmospheric effects. 

In 1883, an Indonesian volcano named Krakatoa exploded. Scientists likened the blast to a 100-megaton nuclear bomb. From 600 km away, people heard the noise as loud as a cannon shot. Plumes of ash rose to the very top of Earth's atmosphere and the collection of that ash made the Moon look a bluish color.

Some of the ash-clouds were filled with particles about 1 micron (one millionth of a meter) wide, which is the right size to scatter red light, while allowing other colors to pass. White moonlight shining through the clouds emerged blue, and sometimes almost green.

Blue moons persisted for years after the eruption.

People also saw lavender suns and, for the first time, noctilucent clouds. Other less-potent volcanic eruptions have caused the Moon to look blue, too. People saw blue moons in 1983, for instance, after the eruption of the El Chichón volcano in Mexico. There were also reports of blue moons caused by Mt. St. Helens in 1980 and Mount Pinatubo in 1991.

So, will you ever see an blue Moon? In astronomical terms, it's almost guaranteed you will see one if you know when to look. If you hope to see a full Moon which is the actual color blue, that is less likely. But it is possible, especially during forest fire season.

Edited by Carolyn Collins Petersen.